Quantum Entanglement Device and Method of Manufacture

This application claims the benefit of and priority to U.S. Provisional Patent Application No. 63/364,840 filed May 17, 2022, entitled “QUANTUM ENTANGLEMENT DEVICE AND METHOD OF MANUFACTURE,” the entire contents of which is hereby incorporated herein by reference.

The present invention relates to the field of quantum physics and, more specifically, to an optical device configured to generate and manipulate entangled photon pairs for applications in quantum communication, quantum computing, and related fields.

Quantum entanglement, a phenomenon in quantum physics, allows for the generation of correlated particle pairs that exhibit non-classical correlations, enabling various applications in quantum information processing. For instance, when a photon is passed through a crystal or optical device that has certain non-linear optical properties, the photon can split into two different photons. These photons can be entangled is various ways, one of them being polarization entanglement. The resulting photons are “entangled,” meaning that a polarization state of a first photon is dictated by the polarization state of the other photon. Beta-Barium Borate (BBO) crystals and the like are used in generating entangled protons. Various challenges exist in increasing the efficiency or likelihood of a generating a pair of entangled photons. Currently, when a photon is passed through a crystal or like optical device, the chances of generating a pair of entangled photons are remote. Moreover, existing methods for generating entangled photon pairs often require complex and expensive setups, limiting their practicality and scalability.

In a first aspect, a method of manufacturing an optical fiber is described. In one example, the method includes providing a fiber preform, heating the fiber preform in a draw tower until the fiber preform reaches a predetermined temperature, and then drawing the preform into an optical fiber. The fiber preform used in the drawing process can be formed or manufactured by heating a crystal material or composition in a suitable crucible which is placed into a furnace and heated to a temperature above the melting point of the crystal material, inserting a glass tube into the heated crucible containing the molten crystal, applying suction to draw the crystal material into the glass tube, and cooling the glass tube and the crystal composition disposed therein. Any number of tubes of crystal material can be formed in a similar way.

One or more such glass tubes containing the crystal material can be inserted into holes or apertures formed in a larger glass tube preform, where the outer diameter of the glass tube containing the crystal matches the inner diameter of the holes in the larger glass tube preform. If more than one crystal region inside the fiber is desired, more tubes containing the crystal material can be inserted into any number of holes in the larger glass tube preform.

Additionally, an optical fiber core rod (e.g., a glass rod with the composition that is desired to be in the core region as is generally known by one skilled in the art) can be inserted into a center hole of the larger glass tube preform, and the glass tubes containing the crystal material can be arranged either symmetrically or asymmetrically around the core rod. In some cases, more than one core rod can be used in conjunction with one or more tubes of crystal material dispersed symmetrically or asymmetrically within the fiber preform.

Different crystal composition materials can be incorporated into the fiber preform by inserting the tubes in the desired location and having the desired crystal composition in each tube. The preform generated in this manner having one or more crystal material regions with one or more cores can be mounted and drawn in a draw tower, thereby generating an optical fiber with at least one photon entanglement media disposed therein. The photon entanglement media can include at least one non-linear crystal, where the at least one non-linear crystal has a predetermined size and a predetermined location in the optical fiber through controlling of size and location of the crystal material in the preform. The drawing parameters of temperature of the preform, speed with which the glass fiber is drawn and cooling rate of the optical fiber (which can be controlled by utilizing a tube furnace below the draw tower furnace), in addition to the crystal material region size and location within the preform, can be varied to control the size of the crystals in the crystal material containing regions which reside inside the optical fiber after drawing.

The at least one non-linear crystal can be a plurality of non-linear crystals. The plurality of non-linear crystals can include a first collection of non-linear crystals at a first location of the optical fiber and a second collection of non-linear crystals at a second location of the optical fiber. The crystals which could be used include but are not limited to beta Barium Borate (BBO) crystals, KTP (KTiOPO), LiO, DKOP, LiNbO, KTA, AgS, CdSe, GaSe, CLBO, Yb:YAG, BiBO, Barium Titanate, silicon crystals, other non-linear crystals, or a combination thereof.

In a second aspect, an optical device for quantum entanglement generation manipulation and detection is described. The device can include a pump photon source module configured to generate photons of the appropriate wavelength, an optical fiber containing crystal material regions, and a photon detection module configured to measure and characterize the entangled photon pairs.

The photon source module is further configured to produce photons of the desired wavelength and within a spectrally narrow bandwidth (tuned to the necessary wavelength for entanglement for the crystal used within the fiber). The source of photons is coupled into the optical fiber which contains the non-linear crystal which can be a Barium Borate (BBO) crystal or other non-linear crystal which is selected from a group containing: KTP (KTiOPO), LiO, DKOP, LiNbO, KTA, AgS, CdSe, GaSe, CLBO, Yb:YAG, BiBO, Barium Titanate, silicon crystals, other non-linear crystals, or a combination thereof. The source photons travelling through the optical fiber interact with the nonlinear crystal material which transforms the source photon at one energy (and wavelength) into 2 photons each with ½ the energy (and correspondingly, double the wavelength). For example, with the use of Barium Borate (BBO) in the fiber, an incoming source photon with a 405 nm wavelength would be split into two photons each with a wavelength of 810 nm. The two photons produced can be polarization entangled, meaning that the polarization state of one depends upon the polarization state of the other.

In this manner, the source photons are converted inside the optical fiber into polarization entangled photons which can be guided inside the optical fiber. It is well known to one skilled in the art that photons can be guided inside an optical fiber. The entangled photons can be guided inside the optical fiber in the same manner that optical fibers are used conventionally today except that the photons are polarization entangled. The optical fiber can be connected to an optical fiber splitter or coupler, which is well known to one skilled in the art, in order to split the optical signal containing the entangled photons into two equal signals, which propagate in the two separate fiber sections of the coupler. The two fiber sections of the coupler can be directed to the detection system. The photon detection module can include a high-performance single-photon detector and associated electronics configured to measure and characterize the entangled photon pairs.

In a third aspect, an optical fiber for generating entangled photons is described. The optical fiber can include an optical core and photon entanglement media disposed relative to the optical core. The photon entanglement media can include at least one non-linear crystal, where the at least one non-linear crystal is positioned in the optical fiber such that a multitude of photon and crystal interactions occur in a single transmission of the photons. The at least one non-linear crystal can be a plurality of non-linear crystals. At least a portion of the plurality of non-linear crystals can be Barium Borate (BBO) crystals. A least a portion of the plurality of non-linear crystals can include KTP (KTiOPO), LiO, DKOP, LiNbO, KTA, AgS, CdSe, GaSe, CLBO, Yb:YAG, BiBO, Barium Titanate, silicon crystals, other non-linear crystals, or a combination thereof.

In a fourth aspect, a method is described, including generating an entangled photon pair using the optical fiber and a photon source described herein, directing the entangled photon pair through a transmission end of the optical fiber, and detecting the entangled photon pair on a receiving end of the optical fiber. The method further includes performing a secure communication using the entangled photon pair. The method further includes performing cryptography using the entangled photon pair. The photon source is a laser configured to emit photons at a predetermined wavelength.

The embodiments described herein relate to a quantum entanglement device and a method of manufacture thereof. Quantum entanglement is a fundamental phenomenon in quantum physics that describes the correlation between two or more particles, such as photons, that are inextricably linked, regardless of the distance between them. It has numerous applications in fields such as quantum computing, cryptography, and communication. Existing methods for generating entangled photon pairs often require complex and expensive setups, limiting their practicality and scalability. As such, there is a need for an improved optical device that simplifies the generation and manipulation of entangled photons while maintaining high levels of entanglement fidelity and photon pair purity.

Even further, certain crystals, such as Barium Borate (BBO) crystals, may be employed to produce quantum entangled photons. When a photon is passed through a crystal or like optical device, the chances, however, of generating a pair of entangled photons are very low. As such, the process is extremely inefficient due to low probabilities of entanglement upon interaction of a photon with the crystal. However, if a beam of transmitted photons were transmitted and placed back through the crystal or like optical device, the probability of generating an entangled photon pair is increased.

Accordingly, the present disclosure provided various embodiments for an optical device that facilitates the generation and manipulation of entangled photon pairs. For instance, in various embodiments, a desired type of crystal or multitude of crystals, such as a BBO crystal or crystals, are placed into or otherwise positioned in an optical fiber. In some embodiments, one or more crystals are placed in a core or in a cladding region to allow light to interact a large number of times with a crystal or a multitude of crystals. A single large crystal or a very large number of tiny crystals may be incorporated into the optical fiber to allow extremely large numbers of photon and crystal interactions to occur. Various configurations may be achieved between multiple crystal regions and multiple optical core regions for further optimization.

Further, in various embodiments, an optical device is described that is a compact and robust apparatus that incorporates advanced optical components, including photon sources, wave plates, beam splitters, and detectors. The optical device generates highly entangled photon pairs with enhanced efficiency, purity, and stability. Moreover, the device incorporates features for precise control and manipulation of entanglement properties, such as entanglement swapping, entanglement purification, and entanglement distribution over long distances.

Additional components of the quantum entanglement device are to be described, followed by a discussion of manufacturing of the same.

Turning now to the drawings,illustrate sectional views of quantum entanglement devices, where the quantum entanglement devices may include a waveguide, such as a quantum entanglement optical fiber, referred to herein as an optical fiber for short. Particularly,illustrates a quantum entanglement optical fiberA,illustrates a quantum entanglement optical fiberB,illustrates a quantum entanglement optical fiberC, andillustrates a quantum entanglement optical fiberD (collectively “optical fibers”). The optical fibersare provided as representative examples in-ID, although other types of waveguides can be employed. The optical fibers, and the features or elements of the optical fibers, are not necessarily drawn to scale in. In some cases, the optical fiberscan include additional elements or features as compared to those shown. In other cases, the optical fiberscan omit one or more of the elements or features shown. Sectional views of the optical fibersare shown in, and the optical fiberscan be manufactured to a range of lengths (e.g., lengths extending into and out of the page) according to the concepts described herein.

Referring first to, the optical fiberA includes regions of photon entanglement mediaA,B, andC (collectively “regions of photon entanglement media” or separately “regionA,” “regionB,” and “regionC”), a central core region, and cladding, among possibly other components. The photon entanglement mediamay include, for example, a single crystal or a multitude of different types of crystals formed or otherwise incorporated into the optical fiberA by controlling time exposure and/or temperature exposure, as will be described. The optical fiberA may allow for large numbers of photon-to-crystal interactions to occur.

The crystals in the regions of photon entanglement mediacan be the same among the regionsA-C in one example. In other examples, crystals among the regions of photon entanglement media can be different among the regionsA-C. In each of the regionsA-C, the crystals may include non-linear crystals, such as BBO, KTP (KTiOPO), LiO, DKOP, LiNbO, KTA, AgS, CdSe, GaSe, CLBO, Yb:YAG, BiBO, Barium Titanate, silicon crystals, other non-linear crystals, or a combination thereof. A non-linear crystal is a material that exhibits non-linear optical properties, meaning its refractive index changes with the intensity of light passing through it. As such, non-linear crystals generate entangled photon pairs through spontaneous parametric down-conversion (SPDC). Common examples of non-linear crystals used for this purpose include BBO, KTP, and so forth. Each of the regionsA-C can include the same type of crystal in one example. In another example, each of the regionsA-C can include a different type of crystal as compared to each other. In still other examples, two or more of the regionsA-C can include the same type of crystal, and other regionsA-C can include a different type of crystal.

Generally, a large number of different configurations can be achieved between multiple crystal regions and multiple optical core regions for further optimization. For example, the optical fiberscan include less than or more than three regions of photon entanglement media. Some of the potential configurations are shown in. In, the optical fiberA includes three regions of photon entanglement mediaincorporated in a circular region or arrangement around a central core region. The central core regionmay include an optical core, as may be appropriate. The central core regioncan be embodied as a cylindrical region of glass or plastic material suitable for guiding light in the optical fiberA. In general, any number of regions of photon entanglement mediacan be incorporated in the fiberA and these may either be symmetrically disposed around the central core regionor not symmetrically arranged. The claddingaround the regions of photon entanglement mediaand the central core regioncan be embodied as a glass or other suitable cladding material.

shows an optical fiberB having differing sizes of the regions of the photon entanglement mediaand, in general, a variety of distributions of these sizes may be employed. For example, the region of photon entanglement mediaA is larger in diameter than the region of photon entanglement mediaD. Additionally, the optical fiberB includes a greater number of the regions of photon entanglement mediathan the fiberA shown in.

shows regions of the photon entanglement mediaarranged in rings around a central core region.illustrates an optical fiberD including an optical core, such as pure silica or germanium-doped silica, in the center of the optical fiberD with the photon entanglement media arrangedin a random fashion therearound.

In addition to a core, the optical fibersof the embodiments ofmay include cladding, a coating (not shown) surrounding the cladding, a strength member (not shown), an outer jacket (not shown), as well as other components understood in the art as being included in optical fibers, as may be appreciated. The claddingmay include glass cladding or other suitable cladding according to various embodiments. The glass claddingmay provide a lower reflective index than the optical corein some embodiments.

In general, a variety of materials and/or structures that are used commercially for forming core regions of optical fibersmay be incorporated into the optical fiberas the central core regionor as the optical core, so as to minimize the loss of an optical signal propagating in the core region. The optical fibersmay be designed such that an evanescent field emanating from the central core regionor the optical coreinto the claddingcan be used to allow interaction of light with the photon entanglement mediain the cladding.

In various embodiments, the size (i.e., sectional diameter) of the regions of photon entanglement mediamay be in a range from hundreds of microns down to a few nanometers, although other dimensions may be employed. A number of the regions of the photon engagement mediamay be larger (e.g., greater than tens of thousands of microns) for very small crystals. Accordingly, the size and size distribution, location, and/or composition of the photon entanglement media, ordering level, and number of regions can be controlled over a very wide range depending, for instance, on a desired application.

The optical fibersor like quantum entanglement device may be employed in a variety of different applications, from sensing applications to imaging applications to communications to cryptography. To date, there is no easy and efficient way to generate a large number of entangled photons in a device. The optical fibers, however, can be configured to generate and manipulate entangled photon pairs for applications in quantum communication, quantum computing, and related fields.

Accordingly, various embodiments for an optical fiberor other like quantum entanglement device are disclosed that may incorporate a variety of desired crystals such, as BBO, into an optical fiber either in a core or in cladding, to allow light to interact a large number of times with the crystal or crystals. Example fibers which have been produced with a barium borate core, as disclosed in the embodiments described herein, are shown in. Specifically, a micrograph ofshows a composite picture of many different fiber cross-sections with different core sizes and different claddingglass diameters.are enlarged photographs of some of these fibers.

includes a flowchartthat describes an example process for manufacturing one or more of the optical fibersshown inor like photon entanglement devices. In addition, the process of the flowchartofdescribes controlling a size and/or location of a crystal (or collection of crystals) in optical fibersor like devices. For instance, an optical fibermay be formed having a first collection of crystals at a first location of the optical fiber, a second collection of crystals at a second location of the optical fiber, a third collection of crystals at a third location of the optical fiber, and so forth, where the collections of crystals and the locations may be different from one another.

At box, the process includes providing or otherwise forming a fiber preform. The fiber preform can be used to form an optical fiber according to the embodiments described herein, such as one of the optical fibersA,B,C, orD, among others.

At box, the process further includes melting one or more crystals or crystal materials. The crystals can include non-linear crystals, such as BBO, KTP (KTiOPO), LiO, DKOP, LiNbO, KTA, AgS, CdSe, GaSe, CLBO, Yb:YAG, BiBO, Barium Titanate, silicon crystals, other non-linear crystals, or a combination thereof. The crystal or crystals can be melted in a crucible in a furnace, for example, to a temperature above the melting point of the crystals. Other suitable techniques can be relied upon to melt the crystal or crystals to a temperature above the melting point. Depending on the type of optical fiber being formed, the process can include melting a number of different crystal materials separately.

At box, the process further includes suctioning the melted crystal material or materials into one or more tubes. The tube or tubes can be glass tubes in one example. The tubes can have the same or different outer diameters (“ODs”). The tubes can ultimately form part of the cladding of the optical fiber being formed in the process. The tube or tubes can be inserted, at one end, in the melted crystal material or materials. Suction can be applied to the tube or tubes, at another end, to draw the melted crystal material or materials into the tube or tubes. After the crystal material or materials have been drawn up into the tube or tubes, the tubes and crystal materials can be cooled. These tubes of crystal materials can be used to form the regions of photon entanglement mediain the optical fibersshown in, for example. Tubes having a range of different ODs can be used to form regions of photon entanglement mediahaving different cross-sectional diameters.

At box, the process further includes inserting the tube or tubes containing the crystal material or materials into a larger glass tube preform. The glass tube preform can have one or more holes or apertures formed in it, at locations in which the regions of photon entanglement media are to be positioned in the resulting optical fiber. The glass tube preform can have any number of holes formed in it, at various locations. The holes can be arranged symmetrically, such as in a ring or circle, or asymmetrically or randomly, consistent with the examples described above in. Each of the holes can have an inner diameter (“ID”) that is sized large enough for insertion of one of the tubes containing the crystal material or materials. The glass tube preform can thus include a number of different holes or apertures having different IDs, each matching (although larger than) the ODs of the tubes containing the crystal material or materials.

The tube or tubes containing the crystal material or materials can be inserted into the holes within the larger glass tube preform at the desired locations. Additionally, an optical fiber core rod can be inserted into a central hole within the glass tube preform, to form the central core region. This optical fiber core rod can ultimately form the central coreor the optical coreof an optical fiber, consistent with the examples described above in.

At box, the process can include heating the fiber preform formed at boxto a predetermined viscosity, to a predetermined temperature, or to both a predetermined viscosity and temperature. In one example, the fiber preform can be placed into a draw tower for forming fibers and heated in the draw tower. At box, the process can include drawing the fiber preform, after heating, into an optical fiber. Here, the fiber preform can be pulled at one end, for example, and stretched or drawn out into the optical fiber. The drawing process can be facilitated to some extent by gravity, although the fiber preform can also be pulled or stretched. The fiber preform can be pulled to a suitable length for the desired application for the resulting optical fiber. The optical fiber can be one of the optical fibersA-D shown in, as examples.

Turning now to, a schematic diagram of an optical devicefor quantum entanglement is shown according to various embodiments. Generally, the optical devicecan include a photon source module, a photon manipulation module, and a photon detection module.

The photon source modulecan utilize nonlinear optical processes, such as spontaneous parametric down-conversion or four-wave mixing, to generate entangled photon pairs with desired characteristics in an optical fiber comprising a non-linear crustal material disposed within. The photon source modulecan generate entangled photon pairs in one of the optical fibers described herein, such as one of the optical fibersA,B,C, orD, as examples. The photon manipulation modulecan incorporate various optical elements, such as wave plates, polarization controllers, beam splitters, and any combination thereof, to modify and control entanglement properties of generated photon pairs. The photon detection modulecan include high-performance single-photon detectors and associated electronics for efficient measurement and characterization of the entangled photons in the optical fiber.

In addition to the optical device, a quantum entanglement system can include a photon source, a pump laser (e.g., an ultraviolet diode laser), a control and stabilization system, among other devices. The photon source can include a laser or other suitable light source that provides necessary light input for the quantum entanglement process. The laser can emit photons at a predetermined wavelength, which determines the energy and properties of the entangled photon pairs generated. The wavelength as emitted by the laser can be preselected based on a type of non-linear crystal or crystals in the optical fiber. The pump laser is a high-intensity laser that stimulates non-linear crystals, initiating a SPDC process. For instance, the pump laser can generate photons at a specific frequency that matches the phase-matching conditions of the non-linear crystal or crystals in the optical fiber.

In some implementations, if the polarization of a pump beam generated by a pump laser and the axis of the BBO crystal are matched in a way enabling energy and momentum conservation, a portion of the pump photons can be converted into two lower energy near infrared photons at 810 nm. These photons then emerge at opposite sides of an emission cone and form an entangled photon pair.

Phase matching can be performed to ensure efficient SPDC and the generation of entangled photon pairs. Phase matching can thus include matching the propagation velocities and phase velocities of the photons. Proper alignment and control of orientation, temperature, and other factors of a non-linear crystal can be employed to achieve phase matching.

To ensure the stability and reliability of the entanglement generation process, precise control of environmental factors such as temperature, vibration, and electromagnetic interference can be achieved in the quantum entanglement system. Active stabilization techniques can further be employed to maintain the alignment and phase matching conditions throughout the experiment. By combining the foregoing components and carefully controlling their parameters, quantum entanglement in an optical fibercan be achieved using non-linear crystals. This enables applications such as long-distance quantum communication, quantum cryptography, and quantum networking.

For a quantum entangled photon generating fiber, an optical fibercan contain second order nonlinear crystallinity able to interact with light propagating through the optical fiber.

As described above, a process to form BBO crystals inside an optical fiberis described for getting solid BBO inside a fiber preform. To do this, a nonlinear crystal can first be heated up to its melting temperature of approximately 1100° C. inside an alumina crucible for liquification. Fused silica tubes can thus be produced with liquid BBO pulled up through the tube cavity to be later set in a fiber preform for drawing. This can be performed by attaching one end of the glass tube to a low powered vacuum and the other submerged inside the molten pool of BBO.

The liquid BBO, due to the vacuum, can be forced into the tube cavity to begin crystalizing. The tubes containing the crystal material can be inserted into a larger glass tube with a hole that matches closely the diameter of the crystal material containing tube. One or more tubes can be positioned in the larger tube. Each tube can contain the same or different crystal material. The larger tube can also contain one or more optical core regions. An optical fiber can then be made from the preform setup using a fiber draw tower while demonstrating relative ease for the actual draw process comparable to that of standard telecom fibers.

In accordance with various methods described herein, optical fiberscontaining BBO were fabricated and then characterized. From SEM imaging and an energy dispersive spectral analysis, shown in, it can be determined that barium is present in the rings surrounding the core and in the core itself of both fiber samples. The presence of these elements indicates that during the draw process, second order nonlinear BBO crystals permeate the lighter colored core and lighter colored patterns throughout the samples as shown in.

Based on the drawing process and kinetics of the BBO crystallization, it can be determined that there is a wide distribution of second order nonlinear BBO crystal sizes and orientations scattered along the optical path of the optical fiber. Inducing a strain access during the draw process or creating an electrical field perpendicular to the draw axis are potential methods to align crystal orientation if uniformity is desired. Controlling the cooling temperature during the fiber draw process can produce larger or smaller crystal sizes as desired. In this manner a very large number of crystals can be produce throughout the length of the fiber in order to provide a long interaction pathlength for the source photons to interact with the crystals.

Due to the large distribution of crystals, it can be expected that polarized light propagating through the core of the fiber can show a certain probability for spontaneous parametric down conversion. If the polarization orientation of the light aligns with the fast axis of the BBO crystal at any point along the length of the BBO fiber, biphotons then have the possibility of being generated.

Additional embodiments of the present disclosure further include determining crystallization to maximize biphoton generation and locating preferred polarization directions. The probability of quantum entanglement can increase with the length of the fiber as there will be correspondingly more interactions between propagating photons and the BBO crystals. Additionally, the fiber can be drawn so that a specified shorter length on the order of decimeters contains BBO crystals while the rest of the fiber could be standard telecom allowing for long lengths of transmission.

Thus, BBO and other nonlinear crystals have shown success in producing biphotons in free space crystals. A laser beam can be directed through free space to the crystal surface by a series of mirrors, lenses, and/or other optical components. If successful, the higher energy incoming photon can be split into two lower energy photons which are polarization entangled. However, such processes can be extremely inefficient due to the low probabilities of entanglement upon interaction of a photon with the crystal and such processes require precise alignments and space to setup.